Communications system and method

ABSTRACT

A communications system ( 1 ) includes a plurality of nodes ( 2 ). Each node ( 2 ) has receiving means for receiving a signal transmitted by wireless transmitting means; transmitting means for wireless transmission of a signal; and, means for determining if a signal received by said node ( 2 ) includes information for another node ( 2 ) and causing a signal including said information to be transmitted by said transmitting means to another node ( 2 ) if said received signal includes information for another node ( 2 ). Each node ( 2 ) has one or more substantially unidirectional point-to-point wireless transmission links ( 3 ). At least some of the nodes ( 2 ) have plural substantially unidirectional point-to-point wireless transmission links ( 3 ). Each of said links ( 3 ) is to one other node only ( 2 ). Each node ( 2 ) is arranged such that transmission or reception of a signal at any particular frequency by a node ( 2 ) takes place on only one link ( 3 ) at a time.

This is a Division of U.S. application Ser. No. 09/331,219 filed on Jun.17, 1999, now U.S. Pat. No. 6,553,020, which is a U.S. national phase ofPCT/GB97/03472 filed on Dec. 18, 1997, the entirety of which is herebyincorporated herein by reference.

The present invention relates to a communications system and method.

There is an increasing demand for high bandwidth communications systemswhich can carry data at rates which are significantly higher than thosewhich are presently available to business or residential users. Systemswhich would benefit from very high data transfer rates includevideo-on-demand, video conferencing and video “telephony”, business andhome Internet access, local area networks (LAN) interconnects, virtualprivate networks, teleworking, on-line games, high definitiontelevision, and many other applications demanding high informationtransfer rates.

In a conventional telephone communications system, the system operator'smain switched trunk network is connected to an access network whichconnects the trunk network to a subscriber's individual telephonehandset or private branch exchange (PBX). The access network is oftenknown as the “local loop”.

The vast majority of local loop networks in the United Kingdom and manyother countries are based on wires which are either buried in the groundor are suspended overhead from poles. The wire extends from the regionalaccess switch to the subscriber and is essentially dedicated to onesubscriber and carries signals for no-one else.

Copper wire has conventionally been used primarily because of itsrelative low cost. However, copper wire can only carry data at a rate ofabout 2,400 to 9,600 bits per second (bps) without data compression.With more sophisticated techniques, this limit has been increased toabout 57,000 bps. However, this is extremely slow when compared with therate required for real-time video, which is in the region of 2 to 9million bps (Mbps).

Some UK operators are now offering digital access services using theintegrated services digital network (ISDN) system. However, the datatransfer rate is still only about 64,000 to 128,000 bps with ISDN orISDN2 and wired technology is still used. More recently, wired systemssuch as HDSL (high speed digital subscriber line) and ADSL (asymmetricdigital subscriber line) can deliver up to 2,000,000 bps (2 Mbps).However, as these are still wired systems, there is a very substantialstart-up cost for any such system in that the operator must incur thesignificant cost of digging up roads, pavements, etc. to lay the cablesor wires to a large number of subscribers before the system can beginoperating. Indeed, the operator must take a large financial risk whensetting up a new wired system in that the operator must lay a very largenumber of cables or wires before potential customers have committedthemselves to the system so that the operator can offer a system whichis already functional. This is obviously a significant risk,particularly where new technology is involved and the level of customertake-up of the system is unknown at the time the operator installs theinfrastructure for the system.

Similarly, in a conventional, point-to-multipoint (broadcast) cellularsystem, each subscriber unit deals only with information intended forthat subscriber.

Both the standard telephone system and cellular system mentioned aboverequire some form of central station sending information to andreceiving information from outlying or peripheral subscriber stations.

A wireless system is very much cheaper to install as no mechanicaldigging or laying of cables or wires is required. User sites can beinstalled and de-installed very quickly. Thus, radio communicationssystems have many attractive features in the area of large-scale systemdeployment. However, it is a feature of radio systems when a largebandwidth (data transfer rate) is required that, as the bandwidth whichcan be given to each user increases, it is necessary for the bandwidthof the radio signals to be similarly increased. Furthermore, thefrequencies which can be used for radio transmission are closelyregulated and it is a fact that only at microwave frequencies (i.e. inthe gigahertz (GHz) region) or higher are such large bandwidths nowavailable as the lower radio frequencies have already been allocated.

The problem with microwave or higher frequencies is that these radiofrequencies are increasingly attenuated or completely blocked byobstructions such as buildings, vehicles, trees, etc. Such obstructionsdo not significantly attenuate signals in the megahertz (MHz) band butbecomes a serious problem in the gigahertz (GHz) band. Thus,conventional wisdom has been that microwave or higher frequencies aredifficult to use in a public access network which provides communicationwith a large number of distributed users.

The spectral efficiency of any wireless communications system isextremely important as there are many demands on radio bandwidth. As amatter of practice, the regulatory and licensing authorities are onlyable to license relatively narrow regions of the radio spectrum. Acellular system, which uses point-to-multipoint broadcasts, places highdemands on the radio spectrum in order to provide users with asatisfactory bandwidth and is therefore not very efficient spectrally.

The use of repeaters or relays to pass on data from one station toanother is well known in many applications. However, in each case, suchrepeaters broadcast signals, in a point-to-multipoint manner, and aretherefore similar to a cellular approach and suffer from a correspondinglack of spectral efficiency.

According to a first aspect of the present invention, there is provideda communications system, the system comprising a plurality of nodes.Each node has receiving means for receiving a signal transmitted bywireless transmitting means; transmitting means for wirelesstransmission of a signal; and, means for determining if a signalreceived by said node includes information for another node and causinga signal including said information to be transmitted by saidtransmitting means to another node if said received signal includesinformation for another node. Each node has one or more substantiallyunidirectional point-to-point wireless transmission links. At least someof the nodes have plural substantially unidirectional point-to-pointwireless transmission links. Each of said links is to one other nodeonly. Each node is arranged such that transmission or reception of asignal at any particular frequency by a node takes place on only onelink at a time.

Wireless transmission is used to provide communication with each node.In practice, each node is likely to be equipment associated with a userof or subscriber to the system. Each node is preferably stationary orfixed. The nodes operate in a peer-to-peer manner, which is in contrastto the central-master/peripheral-slave manner of say a cellularbroadcast system. In the present invention, information is typicallytransferred in a series of “hops” from node to node around the systembetween a source node and a destination node. In the preferredembodiment, the nodes are logically connected to each other by pluralpoint-to-point links between each linked pair of nodes and can beregarded as providing an interconnected “web” covering a geographicalarea and providing a non-cellular network. The links are substantiallyunidirectional, i.e. signals are not broadcast but are instead directedto a particular node with signals being capable of being passed in bothdirections along the link.

It will be appreciated that some prior art systems have nodes which cancommunicate with each other with the nodes acting as simple repeaters.However, the individual transmissions in such prior art systems areoften omnidirectional or use wide-angled transmission sectors and sosuch systems are still fundamentally cellular in structure. Such priorart systems thus tend to use point-to-multipoint transmissions, using amaster/slave or central/peripheral architecture. In the preferredembodiment of the present invention, the nodes are connected in apeer-to-peer manner, with point-to-point links, in an interconnectedmesh. In the present invention, many links across the system or networkmay be “active”, that is carrying signals, at the same time so thatplural pairs of linked nodes may be communicating with each othersubstantially simultaneously. In the preferred embodiment, for eachnode, only one link is “active” at any one time and the link is activein only one direction at a time (i.e. a node is either transmitting onlyor receiving only on that link). In other words, if a node istransmitting or receiving on one of its links, it will not be receivingor transmitting on any of its other links. This greatly increasesspectral efficiency compared to a cellular system or other systems usingbroadcast transmissions from a node. This configuration also helps tokeep down the cost of the individual nodes as each node only requiresone transmitter and one receiver.

Each node of the invention may be autonomous with respect to, forexample, the transmission of signals to other nodes and need not bereliant on control signals from some central controller or any othernode. “Calls” between nodes can be effectively asynchronous and a callbetween a pair of nodes can start and finish effectively at any time,substantially independently of the state of any other call.

In an example of the invention, each node is a subscriber unit which canbe mounted on or near a subscriber's house. In addition, further nodesmay be strategically placed in other suitable places according to therequirements of the operator. Thus, it is not necessary to provide metal(e.g. copper) wire, fibre optic or other fixed “hard” links to eachuser, which saves the very high costs of digging up roads, laying fixedcables, etc. This means that the entry cost for a provider of the systemcan be relatively very low. A small system providing access for say ahundred or a thousand users can be set up very cheaply and additionalusers can be added later as demand grows.

In contrast to conventional point-to-multipoint broadcast radio systems,the present invention does not require a central transmitter with anextremely high bandwidth to service the subscribers' data demands. Infact, except for possible interfacing to a trunk network, no highcapital cost, high-profile, high-complexity sites are required forair-side interfacing, switching and transmission. These functions can bedelocalised over the whole network in the system described herein.Moreover, the present invention does not require the large and unsightlyradio masts/towers which are typical of cellular systems.

Nodes, as well as carrying traffic intended for other nodes, can also bethe origination and termination point of users' traffic. This hasbenefits for expansion of the network because, in principle, traffic canbe injected and extracted from any node in the network, unlike cellularsystems where a high-profile location (such as a hill top) has to bechosen for this purpose for example.

One or more nodes may be associated with plural users of or subscribersto the system. For example, a small business may have one node to whichtheir internal LAN (local area network) is connected whereby all of theLAN users can access the communications system. A node with a bandwidthof say 2 Mbps could support up to 200 users each requiring a bandwidthof 9,600 bps.

Each node is used to pass on or “route” those signals which includeinformation intended for other nodes in the system. If a node shouldfail in the system of the present invention, there is a loss of serviceonly for the subscriber (if any) associated with that node andinformation for other nodes can be routed through nodes other than thefailed node in the preferred embodiment.

Information is passed as necessary in a series of “hops” from one nodeto another via a preferably predetermined route until the informationreaches its destination node.

The nodes are preferably linked so as to form plural transmission pathloops thereby to provide plural choices of path for the transmission ofa signal between at least some of the nodes. Each loop preferablyconsists of an even number of links. This allows for propersynchronisation of transmission and reception between nodes.

For each node that has plural links to other nodes, each of said plurallinks to another node is preferably associated with a time slot. Eachlink for each node may be associated with a distinct time slot. Thus,where TDM (time division multiplexing) is used, no node has more thanone link having the same time slot number in the TDM frame structure.

The allocation of time slots to the links may be varied such that a linkmay selectively be associated with more than one time slot. This allowsthe effective bandwidth supported by a particular link to be increased,perhaps temporarily, as required by a user associated with a particularnode for example.

Each node preferably has a direct line-of-sight link with at least oneother node such that each node can transmit a signal to another node inline-of-sight with said each node. It will be understood thatline-of-sight means that the path between two nodes connected by aline-of-sight link is entirely or substantially unobstructed such thatthe path is transparent or substantially transparent to the frequencyused.

“Information” in a signal may be for example software, whether for theoperation of the node itself or for use by a subscriber associated withthe node or otherwise, voice telephony data, video data, ortelecommunications traffic generally.

Preferably, a signal including said information is transmitted by a nodeto another node if and only if a signal received at said node includesinformation for another node.

The number of nodes is preferably less than the number of links. Thisserves to ensure that there can be several distinct paths between anytwo nodes. Also, because the traffic equations are under-constrained,the traffic flowing on a link is not only a function of the subscriberinjected/removed traffic, but also a function of the traffic on otherlinks. This leads to a large number of possible traffic configurationsfor any given subscriber traffic. This means that (i) the point-to-pointcapacity of the network is increased relative to chain and treetopologies, (ii) it allows scope for network management strategies toalter traffic flows in parts of the network to prevent congestionwithout, in principle, adversely affecting the traffic carrying-capacityof the network as a whole, and (iii) the spectral efficiency of thesystem can be greatly improved over conventional cellular radiotechniques.

Each node is preferably arranged to be in a transmission mode for a timeperiod which alternates with a time period for a reception mode.

Other duplex techniques, such as Frequency Division Duplex (FDD), may beused.

Because each node is concerned with switching as well as thetransmission of information traffic, the whole system can effectivelybehave as a distributed switch. This means that conventional accessswitches (i.e. exchanges), which represent significant capitalexpenditure, can be eliminated.

Many topologies for connecting the nodes are possible. Possibletopologies include a fully interconnected topology, in which each nodeis directly connected to each other node; a linear chain topology, inwhich each node is connected to two other nodes in a chain; a treetopology, in which each node is connected to a predetermined number ofother nodes such that there are no loops in the topological structure; alattice topology, in which each node is connected to up to apredetermined number of nearest neighbours; and, a hypercube-typetopology in which each node is linked to n other nodes. Non-regulartopologies, with for example a random interconnection of nodes and/or ahigh degree of interconnectivity, are also possible and have manydesirable properties. For example, a non-regular topology (like certainregular topologies) may provide a large number of possible routes forinformation to pass across the system or web. Combinations of topologiesare also possible. For example, a hypercube structure of dimension ncould service clusters of n fully interconnected n-valent nodes. Astructure close to a perfect hypercube could alternatively be used forexample.

It will be appreciated that in most areas where the system is deployed,the location of the nodes is dictated by the subscriber locations andthat lines of sight between the nodes depends on the local geography. Insuch situations, it is unlikely that a prechosen network topology can bemapped onto the available lines of sight. A more pragmatic approach isto build up the network from the available lines of sight, carrying outthe process with a view to creating a network with the desiredtraffic-bearing characteristics. Computer modelling has been carried outand it has been shown that it is possible to fulfil the requirements andpreferred features of the network without having a regular form. Themodelling indicates that structures worked up from the actual physicalconnectivity can perform well with regard to traffic-bearing properties.

Preferably, at least one node is arranged not to transmit to any othernode information in a signal received by said at least one node whenthat information is addressed to said at least one node. Mostpreferably, all nodes operate in this manner.

Each node preferably has addressing means for adding to information in areceived signal the address of a node to which a signal including saidinformation is to be routed when said information is for another node.Thus, each node can easily pass on information intended for other nodes.

The addressing means may include means for determining the route ofinformation through the system and adding an appropriate address to theinformation accordingly.

The nodes may have means for determining the route of informationthrough the system as a whole.

Alternatively, the route of information through the system may bedetermined centrally by a central system controller. Thus, there may beprovided a central system controller for determining the route ofinformation through the system. The system may be used for passingcontrol signals from the central system controller to each node.

At least one node may have means for determining if a received signalincludes information for said at least one node and processing means forprocessing information in a signal addressed to said at least one node.All nodes may operate in this manner.

The transmitting means of the nodes preferably transmit signals at afrequency of at least about 1 GHz. A frequency greater than 2.4 GHz or 4GHz may be used. Indeed, a frequency of 40 GHz, 60 GHz or even 200 GHzmay be used. Beyond radio frequencies, other yet higher frequencies suchas of the order of 100,000 GHz (infra-red) could be used. (The UKWireless Telegraphy Act 1949 defines the upper frequency limit for theradio spectrum as 3×10¹² Hz.) The receiving means are arranged toreceive signals at the frequencies transmitted by the transmittingmeans. It will be understood that, at least from a practical technicalpoint of view, a greater bandwidth is more easily obtained if a higherfrequency is used with suitable modulation.

The link between two nodes may be arranged to use simultaneously two ormore frequency channels. This reduces the bandwidth load on a particularfrequency channel.

The receiving and transmitting means may be arranged to transmit anddetect circularly polarised radiation. The transmitting means preferablyincludes a highly directional transmitter antenna. The receiving meanspreferably includes a highly directional receiver antenna. Each of thesepreferred features helps to prevent interference between nodes and alsohelps to mitigate the effects of multipathing.

All nodes may be substantially identical. This simplifies theimplementation of the present invention and helps to keep down costs.

The system can effectively be a self-contained network. On the otherhand, by way of example, the system may be an access network connectedto a conventional trunk network for providing access to subscribers orto other networks. A further node may be connected by a data connectionto one of the nodes of the system and arranged to transfer a signal toor receive a signal from the trunk network or both.

One or more data storage servers can be connected to or provided atsuitable nodes in the system. Various types of data can be stored onsuch data storage servers. For example, for so-called network computing,a user's software applications can be stored at a data storage serverremote from that subscriber's node. The user accesses those applicationsthrough the system of the present invention. The applications can beeasily updated by the software producer and can be used by pluralsubscribers who perhaps pay the software producer on a time-usage basis.The data stored on the data storage servers could be data for videossuch as films (movies). This would not only provide a distributedvideo-on-demand service, but, in addition, from the system operator'spoint of view, would allow video material to be distributed to theembedded servers using the same system possibly in a broadcast mode. Ineither case, frequently requested material migrates from main systemlibraries out to points in the system where it is required. Thismoderates the bandwidth requirements both for the video servers and foroperator's libraries.

Plural systems, each as described above, can be provided with eachsystem being connected to at least one other system. The connectionbetween such systems can be a radio connection, a wired connection suchas a fibre optic link, or any other suitable means.

At least one link of a node may be arranged to use a first transmissionfrequency and at least one other link of said node may be arranged touse a second transmission frequency. This can be used to help preventinterference between nodes.

In an embodiment, some of the nodes are allocated to subscribers andsome of the nodes are not allocated to subscribers, at least some ofsaid non-allocated nodes being solely for carrying information trafficbetween subscriber nodes.

According to a second aspect of the present invention, there is provideda method of communications. The method comprises the steps of: (A)transmitting a signal from one node to another node along asubstantially unidirectional point-to-point wireless transmission linkbetween said nodes; (B) receiving said signal at said other node; (C)determining in said other node if the signal received by said other nodeincludes information for a further node and transmitting a signalincluding said information from said other node to a further node alonga substantially unidirectional point-to-point wireless transmission linkbetween said nodes if said signal includes information for a furthernode; and, (D) repeating steps (A) to (C) until said signal reaches itsdestination node, wherein transmission or reception of a signal at anyparticular frequency by a node takes place on only one link at a time.

Preferably, for each nodes that has plural links to other nodes, each ofsaid plural links to another node is associated with a time slot, andeach transmission step on a link of said one node occurs during adistinct time slot and each receiving step on a link of said other nodeoccurs during a distinct time slot. The allocation of time slots to thelinks may be varied such that a link is selectively associated with morethan one time slot.

Each node preferably adds to information in a received signal theaddress of a node to which a signal including said information is to berouted when said information is for another node.

Each node may have addressing means, the addressing means determiningthe route of the information through the system and adding anappropriate address to the information accordingly. Alternatively, acentral system controller determines the route of information throughthe system.

The method preferably comprises the step of each node transmitting asignal including said information to another node if and only if asignal received at said node includes information for another node.

The method preferably includes the steps of determining in at least onenode if a received signal includes information for said at least onenode and processing the information in a signal addressed to said atleast one node.

Preferably, the signals are transmitted at frequencies greater thanabout 1 GHz.

There may be at least two possible paths for transfer of data between asource node and a destination node. In such a case, the method maycomprise the step of transmitting a copy of said data on each of said atleast two paths. Alternatively, the method in such a case may comprisethe steps of: transmitting from the source node a part only of said dataon each of said at least two paths and reconstructing the data from saidtransmitted parts of said data in the destination node. This canincrease the effective bandwidth of transmissions and allows redundancyto be achieved.

According to another aspect of the present invention, there is provideda telecommunications switching apparatus, comprising a communicationssystem as described above.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a first example of a systemaccording to the present invention;

FIG. 2 is a schematic representation of a second example of a systemaccording to the present invention;

FIGS. 3 and 4 are schematic representations of further examples ofsystems according to the present invention;

FIG. 5 is a schematic representation of a further example of a systemaccording to the present invention;

FIGS. 6 to 9 are schematic representations of different topologies forthe system of the invention;

FIG. 10 is a schematic illustration of a node showing the radiocomponents;

FIG. 11 is a schematic representation of a time slot structure of a nodetiming frame;

FIGS. 12A to 12C show matrices for explaining the allocation of timeslots to links;

FIG. 13 is a representation of a portion of an example of a systemaccording to the present invention showing synchronism of time slots;

FIG. 14 is a representation of a portion of a further example of asystem according to the present invention showing possible interferencebetween nodes;

FIG. 15 is a schematic representation of a simplified system forexplaining the addressing of signals within a hypercube topology;

FIGS. 16 and 17 show examples of routing algorithms; and,

FIGS. 18 and 19 show examples of connection of systems according to thepresent invention to a trunk network.

In an arbitrary network having a total of N nodes and a total of Einterconnections or links, at each node the traffic flowing into itminus the traffic flowing out of it must be the net traffic introducedby the subscriber associated with that node (neglecting any buffering).If T_(ij) represents the traffic flowing from node i to node j, andB_(i) the user traffic at node i, then the following must be true at anyinstant of time:Σ_(i=0,N) T _(ij) =B _(j), and T _(ij) =−T _(ji), and T _(jj)=0 for j=0to N

Traffic Constraint Equations

Treating the link traffic T_(ji) as unknowns, and the user traffic asknown, there are N+E constraint equations and 2E unknowns, where theexact topology of the network dictates how N and E are related. Thereare two network topology classes of interest for present purposes,namely topologies for which N≧E and topologies for which N<E.

The first type of network topology with N≧E implies that the trafficequations above are completely constrained, i.e. the traffic flowing inany link is completely determined by the known subscriber trafficinjected/removed from the network. Networks of this type can beconstructed by adding only one new link every time a new node is added.Regular forms of such networks are for example one-dimensional chainsand trees (where E=N−1), the topologies encountered in conventionalaccess networks. Another property of these networks is that there isonly one possible route between any two nodes (without traversing anylink twice): there are no loops. Network systems having topologies withN=E may be single chain loops, possibly combined with linear chains andtrees; for these systems, there is a maximum of two paths between anytwo nodes.

The other class of network topology, where the number of possible linksexceeds the number of subscriber nodes (N<E), is of more interest forthe purposes of the present invention. This is for two main reasons.First, there can be several distinct paths between any two nodes.Second, because the traffic equations are under-constrained, the trafficflowing on a link is not only a function of the subscriberinjected/removed traffic, but also a function of the traffic on otherlinks. This leads to a large number of possible traffic configurationsfor any given subscriber traffic. These are highly desirable propertiesbecause (i) the point-to-point capacity of the network is increasedrelative to chain and tree topologies, (ii) it allows scope for networkmanagement strategies to alter traffic flows in parts of the network toprevent congestion without, in principle, adversely affecting thetraffic carrying-capacity of the network as a whole, and (iii) as willbe shown later, the spectral efficiency of the system can be greatlyimproved over conventional cellular radio techniques.

To achieve the above desirable properties, the network is preferablyconstructed such that multiple paths between arbitrary nodes arepossible, i.e. the network contains transmission path loops.

Even in networks in which N<E, connections to trunk networks formpotential bottlenecks where diverse traffic streams are forced through asingle link. This implies that the capacity and location of trunknetwork connections will need to be planned with care. Conventionalaccess networks are dimensioned on the 80/20 rule-of-thumb, that is, byjudicious choice of region, approximately 80% of the traffic generatedby subscribers is confined to that region, with only 20% requiringaccess to the trunk network and this approach can be applied in thepresent invention.

The capacity of the network or “web” depends on how the nodes areactually connected. Consider the example of a network 1 shown in FIG. 1in which each node 2 has a link 3 with its nearest neighbours only. (Itwill be understood that the lines which represent the links 3 betweennodes 2 in the drawings are only schematic and show which nodes 2 areconnected to which other nodes 2 via point-to-point line-of-sightwireless transmissions.) The links 3 between nodes 2 will typically becarrying information not just for the neighbouring nodes but also fornodes further down the path. The amount of bandwidth required for agiven bandwidth ‘delivered’ will depend on the proportion of thebandwidth to be passed on by a node, compared with that being deliveredto the node. This in turn depends on the average number of ‘hops’ that apiece of information has to make to get to its destination. The numberof ‘hops’ taken to get from one node to the next depends on exactly howthe nodes are connected. In the example of FIG. 1, if information is tobe sent between A and O, a route such as ABCDEJO has to be used,requiring a lot of hops. However, if the network were as shown in FIG.2, the route could be ANO, requiring many fewer hops.

Thus, it is desirable to find ways of connecting nodes that minimise thenumber of hops and maximise the number of nodes connected, while at thesame time keeping the number of links per node to a reasonable number.This latter point is important since, trivially, a fully interconnectedweb in which all nodes linked to all others is clearly the best in thatnumber of hops required to transmit between any two nodes is only one,but the number of links per node is equal to the number of nodes and sobecomes large very quickly.

One way of looking at the number of hops (H) problem is to consider theaccess area serviced (A) to be randomly populated with N subscribers. Onaverage, the width of the area will be ≈√A and the mean distance betweensubscribers will be ≈√(A/N). Thus, the number of hops across the regionwill be H≈√N, assuming most nearest neighbours are interconnected. Innetworks of the order of 10⁶ subscribers, this implies 1,000 hops totraverse the network. Given that each hop introduces a finite delay(t_(d)) into the traffic streams retransmitted, it is essential tominimise the product of t_(d) and H. A total end-to-end delay of <50 msis a useful target. For nearest neighbour connectivity, this means thatthe individual node delay must be <50 μs. It is clear that nearestneighbour interconnection schemes will probably give rise tounacceptable traversal delays where the number of nodes is relativelylarge.

A mix of nearest neighbour and more remote point-to-point(line-of-sight) connections may therefore be appropriate. In this way,the number of hops across the network is related more to itsline-of-sight properties than its subscriber density. For example, ifthe mean line-of-sight distance for a particular network is L, thenH≈√A/L, and so if L>√(A/N), the number of hops across the network willbe significantly reduced.

A simple method of ensuring that a system or web 1 of the presentinvention does not have a nearest neighbour-only topology will now bedescribed with reference to FIGS. 3 and 4. As shown in FIGS. 3 and 4,part or all of a web 1 is notionally divided into M (arbitrary)geographical regions of roughly similar populations where M is themaximum valence of a node, i.e. M is the maximum number of links 3 whichcan be supported by a node 2. In the example shown, M is eight. Inpractice, any such geographical division will have to take account oflines-of-sight available. (Note that other nodes 2 in the web 1 andtheir connections have been omitted from FIGS. 3 and 4 for clarity.)

Looking at region A in FIG. 3, it can be seen that the node q in regionA has been connected by a link 3 to other nodes 2 such that no more thanone connection has been made to a node 2 lying in the same region.Connecting all the nodes 2 in this way will clearly ensure that webs 1having nearest neighbour connections only are avoided. Stronger forms ofthis method are possible. For example, connections may be made as above,but which exclude any connection to a node 2 (such as node p) in thesame region. In practice, the exact form of strategy adopted will dependon the geography and the ultimate range of a node 2. Another variant ofthe above scheme, which could be used where node range was restricted,would be to connect only to neighbouring regions, within range, as shownin FIG. 4.

It is important to know what bandwidth is required on each of the linksin order to set up links of bandwidth B between randomly chosen pairs ofnodes until all nodes are connected. Now, to answer this question fullyis complex because it depends on the required traffic characteristicsand the permissible routing algorithms, and would require the generalsolution of the constraint equations above. However, the following givesa simple calculation to find the required bandwidth b of a link tosustain traffic in a web or network, where each node in the web issinking and sourcing bandwidth B. For a network which can cope witharbitrary, symmetric subscriber interconnections, ideally:b≈B  (1)i.e. the required link bandwidth should be independent of the number ofsubscribers in the network and be of the order of the offered traffic ateach node.

Assume that the network is a non-nearest neighbour web, and, as a worstcase, that the data a node is sinking/sourcing is being exchanged withthe most distant node in the network. Since the number of nodes in a webis N, and if each node is sending data to one other node, then there areN connections active. For this web, assume that there are n hops onaverage between a node and the most distant node from it.

The subscriber traffic therefore requires nBN units of bandwidth fromthe network. Now, if the web has E links each of which can carry 2bunits of data (b in each direction), the network therefore has 2bE unitsof bandwidth available. Thus if routing issues are ignored, then2bE=nBN.

Thus, each link carries traffic of bandwidth nBN/2E. If b≈B, thennBN/2E≈B, or nN/2E≈1, so that:n≈2E/N  (2)

Thus the link bandwidth constraint (1) implies a constraint on the meannumber of hops across the web (2) in terms of the number of nodes andlinks comprising the web in that from the point of view of desirablebandwidth properties, the quantity 2E/N should be of the order of themean number of hops across the web.

In a practical system, n should be as small as possible for real-timeservices, as large n means larger transit delays. However, since E/N isrelated to the number of loops possible in the web, this should be aslarge as possible to exploit the desirable properties outlined above. Inpractice, a compromise value must be found.

To examine traffic congestion issues, a symmetry argument together witha simple conceptual routing algorithm for the web may be used. Onesimple routing algorithm specifies that traffic going from one node to asecond node will be split evenly at each intermediary node over each ofthe links leading further towards the destination. This could be doneby, for example, a simple statistical multiplexing scheme. Thus for thefirst half of the journey the traffic is smeared out over the web, andfor the second half the traffic concentrates towards the destinationnode. If only a single connection were active, then with this algorithmthe traffic density would be higher around the two terminal nodes andsparser between them. When all the connections are active, thecontributions to traffic density will tend to average out, depending onthe web symmetry. If there is a high degree of symmetry throughout theweb, the number of traffic “hot-spots” will be minimised and the routingwill tend not to block. Thus, to increase the load-balancing propertiesof the network, it is desirable that the topology be as symmetric aspossible.

It is instructive to consider what the above traffic properties mean ina radio context. If it were possible to create webs of nodes with theabove properties, N nodes could be interconnected with links ofbandwidth B using only a radio spectrum of B Hz (using the simplifyingassumption of one bit per hertz). In fact, for practical reasons, thiscannot be easily achieved (and this is discussed in detail later), butthis property is extremely important as it shows that this structure isfundamentally very much more spectrally efficient than the cellulararchitecture, as will be discussed further below.

A simple practical example of a network or web system 1 according to thepresent invention is shown in FIG. 5. In the example shown, there aresixteen subscribers or users, each of which is associated with a networknode 2. The system 1 is connected via interconnect trunks 4 whichconnect specified nodes 2 to a trunk network 5. Each node 2 has a radiotransceiver unit which is able to transmit and receive high radiofrequency signals, for example at least 1 gigahertz (GHz) or 2.4 GHz or4 GHz or even up to or greater than 40 GHz. The transceiver unit of eachnode 2 is in direct line-of-sight contact with four other similar unitsat other respective nodes 2 by direct line-of-sight links 3. Again itwill be understood that the lines which represent the links 3 betweennodes 2 in FIG. 5 are only schematic and show which nodes 2 areconnected in a point-to-point manner to which other nodes 2 via wirelesstransmissions. It can be seen from FIG. 5 how the nodes 2 of a system orweb 1 according to the present invention can communicate with each othervia other nodes 2 if necessary to avoid buildings 6 or otherobstructions which otherwise block the direct line-of-sight connectionbetween particular nodes 2. It should be noted that each node 2 in thisexample of the system 1 be connected to the same number n of other nodesin a hypercube topology. This results in efficient use of the system 1.However, it is possible for some nodes in the system 1 to be connectedto less than n other nodes in a less-than-perfect hypercube.

As mentioned above and as will be further explained below, a messagefrom any one particular node 2 to any other particular node 2 willusually traverse several links 3 between several nodes 2 in a series of“hops” across the system 1. Each passage of a signal through a node 2produces a delay in transfer of the signal. The delay might be only amillisecond or so, but if there were a very large number of nodes, thisdelay could rapidly build into significant fractions of a second. Suchrelatively long delays would not be generally acceptable in interactiveservices such as voice traffic, video conferencing, etc. Thus, it ishighly desirable to minimise the maximum number of hops required by asignal in transferring across the system 1. For example, the hypercubestructure provides an efficient way of connecting many users with asmall number of maximum hops required to transfer a signal between asource node and a destination node.

Furthermore, each link 3 has a certain fixed information carryingcapacity, determined in large part in practice by the bandwidth of thecarrier signal used to transmit information between nodes 2. Each link 3carries information data intended for a node connected to the link 3 andalso “transit” data intended for other nodes. Indeed, each link 3carries approximately n times the amount of transit data for eachinformation data carried by the link. Thus, it is generally better tohave a relatively small number of links 3 between nodes 2 (i.e. a smalldimension topology) because this increases the bandwidth available toeach message as fewer messages in total have to be carried by each link3.

In a system having a hypercube-type topology, if each node is linked ton other nodes, the maximum number of nodes in such a system, which isequivalent to the maximum number N of users of the system, is 2^(n)where there is just one subscriber per node 2. The maximum number ofhops required to transmit information from any node to any other is n.The total number of links E=n.2^((n−1))=(N/2)log₂ N. There are n!possible topologically equivalent routes for information to cross thesystem, meaning that good service can be maintained for the vastmajority of users even if one or more individual nodes fails for somereason as other routes for messages to cross the system can be found.For example, to service a region of 65,536 users using a hypercubetopology, where, for simplicity, there is one user per node, n=16. Inother words, for a system for 65,536 users, each user node needs to beconnected to 16 other user nodes and a maximum of 16 hops are requiredto transmit information from any one node to any other node in thesystem.

Topologies having a high degree of node interconnectivity support manypossible equivalent routes through the system 1, each having arelatively low number of hops. Node complexity, in terms of the numberof links 3 required by each node 2, scales only very slowly with thesize of the system 1 in a topology such as a hypercube topology. Theratio of user bandwidth to the resultant link bandwidth is low, possiblyless than unity because of the multiple routing possibilities. Nodes 2can be low cost because of the modest bandwidth requirements. The nodes2 can be identical, leading to low installation costs and ease ofoperation, management and maintenance.

The factors which will decide the optimum topology to be used includemessage traffic patterns, geography of the land in which the system isimplemented, user location density, and system application (e.g.video-on-demand or Internet web-browsing).

One alternative topology is a fully interconnected topology shown by wayof example in FIG. 6. Each node 2 is connected to each other node 2 andthus for an N node network, each node 2 must support (N−1) externallinks 3 to other nodes 2. The total number of links 3 is thereforeN(N−1)/2. This topology is most suited to a relatively small number ofnodes 2, for example where N is less than 10. Adding nodes 2 to such asystem 1 means that all existing nodes 2 must be modified tointerconnect to any new node 2. The main advantage of such as system 1is that only one hop is required to transfer a message from any one node2 to any other node 2. Thus, a fully interconnected topology is mostsuited for connecting a small fixed number of nodes 2.

Another alternative topology is a linear chain topology shown by way ofexample in FIG. 7. Each node 2 is connected to two others. In a system 1of N nodes 2, there are thus N links 3 and information will require N/2hops to cross the system 1. Because all message traffic is concentratedonto the chain of links 3, each link 3 must be of high bandwidth(approximately N/2 times the bandwidth required by each node 2), whichmay limit the number of nodes which can be connected in such a topology.A main advantage of such a topology is the comparative simplicity of thenodes 2 which each have only two external links 3.

A further example of a suitable topology is a tree topology as shown byway of example in FIG. 8. In a homogeneous tree topology, every node 2is connected to a fixed number of other nodes 2 in such a way that thereare no “loops”, i.e. there are no paths which can be followed which passthrough the same node 2 more than once. For a tree with nodes 2connected to J other “lower” nodes 2, and having L levels, the number ofnodes 2 is the geometric series:

${\sum\limits_{k = 0}^{L}J^{k}} = \frac{1 - J^{L + 1}}{1 - J}$which for large J tends to J^(L). A disadvantage of this topology isthat at each hop away from a node 2, the nodes 2 must service J timesthe peak bandwidth of the node connection, implying greatly increasedbandwidth requirements on descending the tree. Another disadvantage isthat the nodes 2 differ between levels as they must function differentlymeaning that a system provider must deploy and manage different nodesfor each level. However, an advantage is that at most two hops arerequired to transmit a message from any node 2 to any other node 2 inthe same level (for example, the lowest level in FIG. 8).

An inhomogeneous tree topology relaxes the requirement for the number ofconnected lower nodes 2 to be fixed, though in other respects is similarto the homogeneous tree topology described above.

A yet further example of a suitable topology for connecting the nodes 2is a lattice topology shown by way of example in FIG. 9. Nodes 2 areconnected in an arbitrary manner to up to a fixed number n of nearestnodes 2. In a grid structure a portion of which is shown in FIG. 9,where n=4 and serving say N=10,000 nodes 2, a message may require √N=100hops to cross the system 1, which may lead to unacceptable traversaldelays. Also, the bandwidth requirements of each link 3 may be high asit will be approximately (√N)/2 times the bandwidth required by eachuser.

It will be appreciated that whatever topology is selected for thesystem, it must be flattened onto the effectively two dimensionalgeography of a geographical region, inevitably leading to a requirementfor some links 3 to be longer than others. With present technology, highfrequency transmitters transmitting say 40 GHz frequencies only have arange of about 500 m to 2 km or perhaps at best up to about 4 or 5 km.There is therefore a problem in providing links 3 between nodes 2 whichare more than about 2 km apart. This can be overcome by limiting asystem to a relatively small number of nodes 2, say 1,024 nodes 2. Sucha system 1 can then be connected to other similar systems 1 of the sameor similar size using a large antenna and radio link, a fibre opticlink, etc. Systems 1 having different topologies can be connected to oneanother.

The network 1 may effectively be a mixture of topologies.

In the preferred embodiment, there are multiple links per nodeorientated in arbitrary directions. This could be achieved with multipleradio systems per node. However, when compared with a typical cellularsystem which would only have one radio system per subscriber, this islikely to make the nodes significantly more expensive than theircellular equivalent. This is especially true when the radios areoperating in the high GHz where this element of the system is likely tobe a significant part of the node cost.

To achieve 360 degree angular coverage at a node, it is possible to useone or more antennas which are steerable either electrically orphysically and which can point in any azimuthal direction, or an arrayof fixed antennas each pointing in a different direction such that anyparticular direction is accessible from one of the antennas.

The exact number M of antennas must be chosen to allow complete angularcoverage without adversely affecting link gain. Note, M may be greaterthan n, the maximum active links per node. However, rather thanproviding M pairs of transceivers at each node, each pair beingcontinuously connected to a single antenna, for cost reasons it ispreferred to use only one transceiver per node and make use of timedivision multiplexing (TDM) and time division duplex (TDD) techniques toconnect the transceiver to an antenna. A node therefore has only onetransceiver pair which must be able to use all M antennas. TDM can beused to time-share the antennas with the transceiver. TDD can be used toalternate the receive/transmit operation of the node radio so that it isnever receiving and transmitting simultaneously. Frequency divisionmultiplexing or code-division multiplexing could be used as alternativesto TDM. Frequency division duplex could be used as an alternative toTDD. Other alternative schemes may be possible.

The basic structure of the radio frequency parts of a node 2 is shown inFIG. 10. A receiver 10 and transmitter 11 are alternatively connected toan M-way switch 12 which conducts radio-frequency (RF) power from and tothe antennas 13.

A simple scheme of scheduling the connection of antennas 13 is shown inthe time slot structure in FIG. 11 for the case M=8. Time is dividedequally into “frames” 20 and each frame 20 is divided equally into atransmit phase 21 and a receive phase 22. The transmit and receivephases 21,22 are themselves divided into equal time slots 23. Each oneof these time slots 23 is used for one link 3 from a node 2. Thus, thenode 2 transmits in one time slot 23 on one link 3 then the next timeslot 23 on the next link 3 and so on, followed by receiving in one timeslot 23 on one link 3 and the next time slot 23 on the next link 3 andso on. Each receive time slot 23 of each node is arranged to be longenough to ensure that there is sufficient time for a signal transmittedfrom other nodes 2 to travel to the node 2 in question and also to bereceived in full at the node 2 in question, particularly to ensure thatthe data packet and any guard bands are received.

In-turn sequencing is not the only possible way of addressing antennas13. The total bandwidth available at a node 2 can be partitioned byallocating more or fewer time slots 23 to an antenna 13, within areceive or transmit half frame 21,22. This is illustrated in thematrices in FIG. 12. The columns of the matrices represent the eightantennas 13 on an example node 2, and the rows eight possiblereceive/transmit time slots 23. A ‘1’ in a cell indicates which antenna13 is active during which time slot 23. A ‘-’ in a cell indicates noactivity of an antenna 13 in a particular time slot 23. The number of‘1’s must not exceed the total number of time slots 23 available.

In FIG. 12A, each antenna has a time slot, so each link can carry 1 unitof bandwidth. In FIG. 12B antenna A0 has two time slots allocated, andhence can carry two units of bandwidth. Antennas A1, A2, and A7 eachhave one time slot allocated, and antenna A4 has three time slotsallocated. Antennas A3, A5, and A6 have no allocated time slots andhence are idle. In FIG. 12C all the time slots have been given toantenna A4. This means that link associated with antenna A4 can carryeight units of bandwidth whilst all the others are idle.

It may be noted that whilst TDM/TDD is used to divide up time betweenlinks 3, this does not imply that the time a link 3 spends active isalso divided into time slots. As each link 3 connects only two nodes 2,there is no need for a further time-division structure, for multipleaccess purposes, on a link 3 for the purposes of the present invention.

Considering now the need for synchronisation of transmission andreception by the nodes 2, if any one of the nodes 2 is transmitting thenall the nodes 2 to which it is transmitting must be receiving. This isonly possible with certain web topologies. Many topologies satisfy thistransmit/receive phasing if all transmission path loops in the web havean even number of sides.

Not only must communicating nodes 2 be transmitting or receiving insynchronism but they must agree on the time slot number that they areusing. Referring to FIG. 13, nodes A and B must both be using the sametime slot for the link 3 between them, say time slot 1 transmit for Aand time slot 1 receive for B. Similarly, A and C must use the same timeslot for the link between them, say time slot 2, etc. However, each node2 may only use each time slot once. In the preferred embodiment, thisrequirement is met exactly throughout the network. Thus, each link 3 inthe network 1 is assigned a time slot number such that no node 2 hasmore than one link of the same time slot number. In addition, it isdesirable to minimise the total number of time slots required. If themaximum numbers of links per node is M, it is clear that at least M timeslots are needed. For any network topology with loops having an evennumber of sides, if M is the maximum node valence of the network, then Mtime slots can be consistently allocated in this network.

It will be appreciated that different groups of nodes 2 may becommunicating with each other at any one time. In other words, differenttransmission paths in the system 1 may be active and carrying traffic atany one time.

Reference is now made to FIG. 14 in which part of a web 1 is shown.Using the above described transmit/receive synchronisation and time slotallocation rules, nodes ABCDEF will not interfere with each other.However, there may be a problem with nodes G and H. This is because thelink between nodes D and C uses time slot 2. Now, the radio signal forthis link will not stop at C, but will continue on and may be detectedby the receiver in node H which will also be receiving during that timeslot using an antenna pointing in a similar direction. In theory it maybe possible to design network topologies which somehow avoid thissituation, but given the complexity of real-world subscriberpositioning, this is likely to be infeasible in practice. The system inpractice should therefore be arranged so that, even though thegeometrical arrangement is as shown, the fact that D's signals aredetectable at H does not cause interference to the signals received at Hfrom E.

This can be achieved by using a set of frequency channels and assigningone of these to each link in the network in such a way that allpotentially interfering links are on different channels. The set ofchannels should be as small as necessary. This requirement for a minimumnumber of frequency channels is related to the beam width of the nodeantenna. For large widths, the area of the interference zone PDQ in FIG.14 is also large and hence there is a greater likelihood of nodes suchas G and H lying in it. Similarly, for small beam widths, the zone areais small, thus containing fewer nodes.

In the example shown, this would mean that link DC is on a differentfrequency channel to link GH. Allocating frequency channels is a complextask. Some system modelling has been done to investigate this issue withthe outcome that the frequency reuse factor is similar to the cellularcase, i.e. somewhere between 6 and 10.

The implication of this on design of the nodes 2 is that the radiosystem must be frequency-agile, re-tuning to a different pre-allocatedchannel on each time slot.

As with all communication systems, individual links 3 are liable tosuffer from interference and damage. Very short timescale problems arehandled by standard means, including Forward Error Correction andre-transmissions. On occasion, a link 3 may suffer problems thateffectively make it useless. However, with a web according to thepreferred embodiment, there will always be a large number of equivalentroutes between any two nodes 2 so the loss of some links 3 can becountered by re-routing the connection.

Link loss occurs on several timescales. In the medium term, a temporaryloss for some seconds or minutes may be caused by large vehicles movingnearby, or perhaps a plume of smoke from a fire. The network will copewith these by re-routing traffic to avoid the problem areas until thelink recovers. On a longer timescale, a link 3 may be lost because lineof sight is being permanently obstructed. This may be caused by newbuilding or tree growth. These losses should be handled at a networkplanning level. As a background activity, the network may constantlymonitor all available lines of sight, (i.e. links 3 between nodes),including those which are not currently being used for subscribertraffic. On a timescale of hours and days, or even minutes or seconds,the network can be automatically reconfigured to use different subsetsof the available lines of sight to optimise operational parameters.

Some subscribers may have very stringent requirements for linkavailability and require high integrity links so that theircommunications are not vulnerable to single point failure. When carryingsuch traffic, multiple paths (m) through the network may be used. Twomethods of operation are possible. In the first, each path carries aduplicate of the subscriber data, so that the receiving node 2 mayaccept data from any active path. This uses up m times the basicsubscriber bandwidth (B) for the connection, but is simple to implement.In the second, each path carries part of the subscriber data (with someadditional parity information) so that the receiving node 2 canreconstruct the data from any m−1 paths received and the parityinformation. This uses in total only αB units of bandwidth (α=parityinformation overhead>1). The second example of method of operation canbe extended to protect against multiple path failures but is morecomplex than the first example of method of operation.

The availability of multiple paths is an inherent property of thepreferred embodiment of a web network 1 of the present invention. Bycomparison, provision of multiple physical paths in a cable or wirebased network is enormously expensive.

In above description, one time slot 23 is used to support all of thebandwidth on a link 3. This maximises the raw data transfer rate;however, it is always important to maintain spectral efficiency.

A general calculation of the spectral efficiency of a network inaccordance with the present invention compared to conventional cellularapproaches is not easy to calculate as much depends on the exactimplementation. However, a cellular approach requires approximately:

-   -   α.N.B_(subs).F_(cell) units of bandwidth, where:    -   α is the maximum fraction of active subscribers    -   N is the number of subscribers    -   B_(subs) is the bandwidth required by a subscriber    -   F_(cell) is the cellular frequency reuse factor,    -   and assuming a modulation technique giving one bit/Hz.

The present invention requires approximately n.B_(link).F_(web) units ofbandwidth, where:

-   -   n is the maximum number of links/time slots on a node    -   B_(link) is the bandwidth of a link    -   F_(web) is the frequency reuse factor needed to minimise        interference in the present invention,    -   again assuming a modulation technique giving one bit/Hz.

F_(cell) is typically in the range 6 to 10 and computer modelling hasshown F_(web) to be much the same. Computer modelling has been carriedout for a number of scenarios and a reasonable set of parameters is thatn=8 and that B_(link) is equal to B_(subs).

This gives the efficiency of the web approach to the cellular approachas:(α.N)/n

For a cell covering 1000 users and a peak active load of 20% (a typicalestimate for video-on-demand services), the relative efficiency is 25fold. This is extremely important as there are many demands on radiobandwidth and as a matter of practice the regulatory and licensingauthorities are only able to license relatively narrow regions of theradio spectrum. The present invention places much lower demands on theradio spectrum than a cellular system providing a comparable userbandwidth.

A simple example of a routing protocol will now be described. The system1 is well suited to the use of asynchronous transfer mode (ATM)technology which can support connection oriented (circuit switched) orconnectionless (packet switched) traffic modes by the transfer of 53byte information “cells”.

In a hypercube topology network with n connections at each node 2, eachout-going connection can be labelled with an index (0 . . . n−1). A paththrough the network system 1 can then be defined by a list of suchindices. As will be understood from the above, the maximum length ofthis list will be n entries.

In general, an information packet can be defined to be of type Messagewhich has two components:

-   -   information cell payload (cell), and    -   the routing address (L).

The routing address is the absolute address of a node 2 in the networksystem 1. Each node 2 will have access to its own address (my_L in thecode discussed below). To see how addressing works in such a system 1,consider the addressing of points on a simple unit 3-cube shown in FIG.15. Each node 2 has a labelled set of channels which can be thought ofas Cartesian axes, in this case X, Y and Z. Thus each node 2 has anX-channel, a Y-channel and a Z-channel.

The address (L) of a node 2 in a 3-cube geometry is one of the eight3-vectors: (0,0,0), (1,0,0), . . . (1,1,1). A move through the cube byone hop along a link 3 (i.e. traversal of an edge) is represented by thefollowing relationship between the initial (L1) and final (L2) address:|L1−L2|=1

Thus, a “forward” move is defined by L1−L2=1 and a “backward” move byL1−L2=−1.

The routing algorithm shown in FIG. 16 replicated in each node 2 of thesystem 1 will in principle ensure correct cell routing.

The function of the handleReturnedMessage function is to takeappropriate action with a returned message. This strategy will depend onthe type of data service supported. It could be one of the following:

1. Return the message to sender, i.e. propagate the message all the wayback to the originator. This should signal to the originator that thereis congestion and that it should pause sending information for a periodof time.

2. Store the message for a period of time, then attempt to forward it toits destination as before.

3. Forward the message forcing a different route to be taken, forexample, by choosing an output channel which has low congestion.

4. Discard the message, assuming that a higher-level data-link protocolwill detect the loss and cause the originator to re-transmit themessage.

The procedure SendPacket (msg, next_node) conceptually sends Message msgto the outgoing link 3 with index next_node. The procedure ProcessCell(cell) is responsible for consuming the information cell locally andmaking it available to the user.

The decideNextChannel function has a functionality which is networktopology specific. For the case of a hypercube topology, an example ofthis is set out in FIG. 17, where ActiveChannels is the number ofcurrently configured channels on a node 2 (which may vary for each node2 in the system), and MaximumChannelUtilisation is the value at andabove which the outgoing channel may be considered to be at fullcapacity and can therefore accept no further traffic.

Where the instantaneous utilisation of an output channel is a measure ofthe traffic loading of that channel over an immediately previous periodof time. Such a measure of traffic loading might be one, or acombination of, the following factors:

1. The number of currently allocated communications circuits on the link

2. The amount of data sent on the link.

In addition, the ChannelUtilisation function may be used to controlnon-existent links as in the case of a partially complete hypercube. Inthis case, the link utilisation could be set permanently toMaximumChannelUtilisation.

A continuously operating function of a node would be the monitoring ofthis loading and allow the routing software to obtain a value related tothe current loading for a given link. This is what the functionChannelUtilisation (channel) does.

The procedure MapWeightedChannelToBestChannel translates the inputweighted channel index into a real output channel for the node. Thesimplest, non-trivial case would be where output channels are denoted byinteger values, 0 to 7 for example, and the mapping of the real weightedchannel number to this is simply a rounding operation. For example,weighted channel value 6.7152 is mapped to channel index 7.

The performance of the system 1 has been primarily described so far interms of its ability to move data within a cluster of nodes 2. However,for many types of service it is required to connect into a trunk network5, as indicated in FIG. 5. For example, in a network of say 250 usersused primarily for 5 Mbps video-on-demand (VOD) service with a loadingof say 0.3 Erlang per user, the total bandwidth required from the trunknetwork is 375 Mbps, assuming that no source material migration takesplace. As the maximum input rate to a node will be say about 40 Mbps(assuming eight links 3 of maximum 5 Mbps each per node 2), this 375Mbit/s of traffic will need to be groomed onto the trunk network at atleast ten locations. This can be done in two ways.

The first alternative is to connect the subscriber interface of a node2′ at each of the “input” locations to a suitable interface on the trunknetwork 5 (e.g. DS3, STM0, 1) as shown in FIG. 18. The nodes 2′ at theinput locations can be connected by an optical fibre 4 to the fibrebackbone of the trunk network 5 for example. These input locations canbe chosen for network deployment convenience rather than by subscriberlocation. This is much easier than running fibre to cellular type basestations where the positions of the base stations are dictated by thecellular structure.

The second alternative is to configure a set of nodes 2″ so that alltime slots 23 are used on one link. This provides several point to pointconnections with exactly the right bandwidth (40 Mbps) for connectioninto a node 2. The specially configured nodes 2″ can be connected by asuitable data connection 6 to a normal subscriber node 2 at the samelocation. It should be noted that these specially configured nodes 2″can use exactly the same hardware as the normal subscriber nodes 2.However, the specially configured nodes 2″ could each use a high gain,long range movable antenna if required. Such antennas could be directedat a cluster 7 of suitably configured nodes 8, located at a single trunkaccess point 9 as shown in FIG. 19.

A problem with many radio communications systems is multipathing. Thiscan occur when a receiver receives a main signal received directly froma transmitter but also receives signals from the transmitter which havebeen reflected from buildings or moving vehicles, for example. Thereflected signal is delayed relative to the main signal, which can leadto cancellation of the main signal if the reflected signal is an oddnumber of half wavelengths lagging in phase. With medium wavetransmissions, where wavelengths of several hundreds of meters are used,cancellation is not much of a problem; the user can usually find aposition for the receiver where cancellation because of reflections frombuildings does not occur or, where cancellation occurs because of asignal reflected from a moving vehicle, the cancellation only occursbriefly and the vehicle moves away, thereby removing the problem.However, at higher frequencies, where wavelengths might be severalmillimeters, objects moving past a receiver can cause frequentcancellation of the main signal by virtue of those moving objectsregularly and frequently reflecting signals which lag the main signal byan odd number of half wavelengths.

In order to overcome this multipath problem should it arise in thesystem 1 of the present invention, it is preferred that the antennas ofthe transmitters and receivers in each node 2 be highly directional.With a highly directional transmitter/receiver, there tends to be bettergain and therefore better signal strength than with an isotropicantenna. Thus, not only does a highly directional transmitter/receivertend naturally to detect only the main signal coming along theline-of-sight link 3 to the node 2 and does not detect reflected signalswhich approach the node at an angle to the main signal, a highlydirectional transmitter/receiver has improved operating characteristicsby virtue of the higher gain available. In addition to thehigh-directionality geometry of the antennas, circular polarisation ofthe transmitted radiation can be used to provide additional protectionagainst loss of signal due to multi-path effects. On being reflectedfrom a surface, a radio wave suffers a change in its phase relative tothe incoming wave. If this incoming wave is right-hand circularlypolarised, for example, then on reflection, this polarisation will bereversed to left-hand circular polarisation. In this way, unwantedreflected radiation is rejected relative to directly transmittedradiation if the receiver is selective to purely right-hand circularlypolarised radio waves. A similar argument would apply if left-handcircularly polarised receivers and transmitters were to be used. Thus,preferably, the system 1 uses line-of-sight, highly directional, highgain, high frequency transmitters/receivers equipped to emit and detectcircularly polarised radiation.

It will be appreciated that in the system 1 of the present invention, nobase transmitter station is required and the system 1 can be constructedfrom a single type of identical transceiver unit at each node 2. Thenetwork system 1 is potentially very much easier and cheaper to build,deploy and maintain in comparison with a cellular system which uses basestations. There is no burying or suspending of cables or wires orerecting of many large base-station antenna masts, again representing alarge saving in costs and also minimising the environmental impact ofthe system 1. The capacity of the system 1 is very large as there aremany possible routes between nodes 2 and to the edge of the system 1.Failure of a particular node 2 accordingly implies loss of service foronly one user and other users are not normally affected as alternativepaths can be found for transmission of a signal. Because each node 2 isconcerned with switching as well as the transmission of informationtraffic, the whole system 1 effectively behaves as a distributed switch.This means that conventional access switches, which representsignificant capital expenditure, can be eliminated.

The present invention allows an operator to begin operating acommunications system 1 having very high data transfer rates to a smallnumber of users at relatively low cost. For example, 128 nodes can beset up in a system as described above at very low cost compared, forexample, to equivalent cable and cellular solutions. Subscribers to thesystem can be allocated respective nodes 2. The remaining nodes 2 whichhave not been allocated to a user can be used as “strategic” nodes 2solely for carrying information traffic between user nodes 2. As moreusers join the system, the strategic nodes can be allocated to the newusers. As the initially implemented system 1 fills so that all nodes 2are allocated to users, new nodes can be added and the system 1 as awhole can be reconfigured to introduce the new nodes to the system. Ifnecessary, a similar process in reverse can be applied to remove nodeswhich are no longer required or which are in maintenance or have failed,for example.

The maximum bandwidth which may be delivered to a node user from thenetwork side (Bdown) and the maximum bandwidth a user may deliver to thenetwork (Bup) may be independently configured dynamically by the networkoperator without affecting the capacity of the node for transit traffic.For example, a low tariff service might be Bup<<Bdown (similarly to ADSLservice), whereas a higher-tariff service might allow Bup=Bdown(‘symmetric’ service). Clearly both Bup and Bdown must be less than thepeak user data rate allowed by the radio system.

A link 3 between two nodes 2 may actually comprise two or more parallelradio channels, i.e. the link 3 uses simultaneously two or morefrequency channels, thus reducing the bandwidth load on a particularradio channel.

The overall control of routing of the signals between nodes can be byvirtue of a central controller. The central controller might perform aperiodic (e.g. daily) check on the system 1 as a whole to determinewhether any nodes 2 have failed. The central controller 1 can thendetermine which route should be followed by a message from any one node2 to any other node 2 in the system 1. Appropriate instructions couldthen be sent from the central controller to each node 2 so that eachnode 2 applies an appropriate address to each information packet.

The present invention allows very high data transfer rates to beachieved. For example, as mentioned, a total node data transfer rate of40 Mbps is entirely feasible. Data transmission rates of 5 Mbps withburst rates of 25 Mbps can be achieved with ease.

An embodiment of the present invention has been described withparticular reference to the examples illustrated. However, it will beappreciated that variations and modifications may be made to theexamples described within the scope of the present invention.

1. A communications system, the system comprising: a plurality of nodes,each node having: receiving means for receiving a signal transmitted bywireless transmitting means; transmitting means for wirelesstransmission of a signal; and, means for determining if a signalreceived by said node includes information for another node and causinga signal including said information to be transmitted by saidtransmitting means to another node if said received signal includesinformation for another node; each node having one or more substantiallyunidirectional point-to-point wireless transmission links, at least someof the nodes having plural substantially unidirectional point-to-pointwireless transmission links, each of said links being to one other nodeonly, and being arranged such that transmission or reception of a signalat any particular frequency by a node takes place on only one link at atime.
 2. A system according to claim 1, wherein the nodes are linked soas to form transmission path loops thereby to provide plural choices ofpath for the transmission of a signal between at least some of thenodes.
 3. A system according to claim 2, wherein each loop consists ofan even number of links.
 4. A system according to claim 1, wherein foreach node that has plural links to other nodes, each of said plurallinks to another node is associated with a time slot.
 5. A systemaccording to claim 4, wherein each link for each node is associated witha distinct time slot.
 6. A system according to claim 4 or claim 5,wherein the allocation of time slots to the links can be varied suchthat a link may selectively be associated with more than one time slot.7. A system according to claim 1, wherein each node has a directline-of-sight link with at least one other node such that each node cantransmit a signal to another node in line-of-sight with said each node.8. A system according to claim 1, wherein each node comprises means fortransmitting a signal including said information to another node if andonly if a signal received at said node includes information for anothernode.
 9. A system according to claim 1, wherein each node is stationary.10. A system according to claim 1, wherein the number of nodes is lessthan the number of links.
 11. A system according to claim 1, whereineach node is arranged to be in a transmission mode for a time periodwhich alternates with a time period for a reception mode.
 12. A systemaccording to claim 1, wherein at least one node is arranged not totransmit to any other node information in a signal received by said atleast one node when that information is addressed to said at least onenode.
 13. A system according to claim 12, wherein each node is arrangednot to transmit to any other node information in a signal received bysaid at least one node when that information is addressed to said atleast one node.
 14. A system according to claim 1, wherein each node hasaddressing means for adding to information in a received signal theaddress of a node to which a signal including said information is to berouted when said information is for another node.
 15. A system accordingto claim 14, wherein the addressing means includes means for determiningthe route of information through the system and adding an appropriateaddress to the information accordingly.
 16. A system according to claim1, further comprising a central system controller for determining theroute of information through the system.
 17. A system according to claim1, wherein at least one node has means for determining if a receivedsignal includes information for said at least one node and processingmeans for processing information in a signal addressed to said at leastone node.
 18. A system according to claim 1, wherein the transmittingmeans of the nodes are arranged to transmit signals at frequenciesgreater than about 1 GHz.
 19. A system according to claim 1, wherein thelink between two nodes is arranged to use simultaneously two or morefrequency channels.
 20. A system according to claim 1, wherein saidreceiving and transmitting means are arranged to transmit and detectcircularly polarised radiation.
 21. A system according to claim 1,wherein the transmitting means includes a highly directional transmitterantenna.
 22. A system according to claim 1, wherein the receiving meansincludes a highly directional receiver antenna.
 23. A system accordingto claim 1, wherein each node is substantially identical.
 24. A systemaccording to claim 1, wherein the system is connected to a conventionaltrunk network for providing access to other networks.
 25. A systemaccording to claim 24, comprising a further node connected by a dataconnection to one of the nodes of the system and arranged to transfer asignal to or receive a signal from the trunk network or both.
 26. Asystem according to claim 1, wherein a data storage server is connectedto or provided at a node.
 27. A system according to claim 1, wherein atleast one link of a node is arranged to use a first transmissionfrequency and at least one other link of said node is arranged to use asecond transmission frequency.
 28. A system according to claim 1,wherein some of the nodes are allocated to subscribers and some of thenodes are not allocated to subscribers, at least some of saidnon-allocated nodes being solely for carrying information trafficbetween subscriber nodes.
 29. A method of communications, the methodcomprising the steps of: (A) transmitting a signal from one node toanother node along a substantially unidirectional point-to-pointwireless transmission link between said nodes; (B) receiving said signalat said other node; (C) determining in said other node if the signalreceived by said other node includes information for a further node andtransmitting a signal including said information from said other node toa further node along a substantially unidirectional point-to-pointwireless transmission link between said nodes if said signal includesinformation for a further node; and, (D) repeating steps (A) to (C)until said signal reaches its destination node, wherein transmission orreception of a signal at any particular frequency by a node takes placeon only one link at a time.
 30. A method according to claim 29, whereinfor each node that has plural links to other nodes, each of said plurallinks to another node is associated with a time slot, and eachtransmission step on a link of said one node occurs during a distincttime slot and each receiving step on a link of said other node occursduring a distinct time slot.
 31. A method according to claim 30,comprising the step of varying the allocation of time slots to the linkssuch that a link is selectively associated with more than one time slot.32. A method according to claim 29, wherein each node adds toinformation in a received signal the address of a node to which a signalincluding said information is to be routed when said information is foranother node.
 33. A method according to claim 29, wherein each node hasaddressing means, the addressing means determining the route of theinformation through the system and adding an appropriate address to theinformation accordingly.
 34. A method according to claim 29, wherein acentral system controller determines the route of information throughthe system.
 35. A method according to claim 29, comprising the step ofeach node transmitting a signal including said information to anothernode if and only if a signal received at said node includes informationfor another node.
 36. A method according to claim 29, including thesteps of determining in at least one node if a received signal includesinformation for said at least one node and processing the information ina signal addressed to said at least one node.
 37. A method according toclaim 29, wherein the signals are transmitted at frequencies greaterthan about 1 GHz.
 38. A method according to claim 29, wherein there areat least two possible paths for transfer of data between a source nodeand a destination node, and comprising the step of transmitting a copyof said data on each of said at least two paths.
 39. A method accordingto claim 29, wherein there are at least two possible paths for transferof data between a source node and a destination node, and comprising thesteps of transmitting from the source node a part only of said data oneach of said at least two paths and reconstructing the data from saidtransmitted parts of said data in the destination node.
 40. Atelecommunications switching device, comprising a communications systemaccording to claim 1.